Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates, including, for example, paper, paperboard stock, corrugated board, films, foils and laminates. Coarse surfaces and stretch films, such as newspapers and grocery bags, can be economically printed by means of flexography.
Flexographic printing elements comprise relief image elements raised above open areas. Such printing elements offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. A typical flexographic printing element as delivered by its manufacturer, is a multilayered article and may include a backing or support layer, one or more unexposed photopolymer layers, a protective layer or slip film, and a cover sheet. As described herein the term “flexographic printing element” encompasses structures in any form suitable for printing, including, but not limited to flat sheets, plates, seamless continuous forms and, cylindrical forms.
Optionally, the element can include an adhesive layer between the support and the photopolymerizable layer, or a surface of the support that is adjacent to the layer of photopolymer has an adhesion-promoting surface. In addition, in some embodiments, the flexographic printing element may include an actinic radiation opaque layer for forming an in-situ mask on the element. If used, the actinic radiation opaque layer may substantially cover the surface or only cover an imageable portion of the layer of photopolymer. The actinic radiation opaque layer is also substantially opaque to actinic radiation that corresponds with the sensitivity of the photopolymer.
The flexographic printing element may further include one or more additional layers on or adjacent to the layer of photopolymer. Examples of additional layers include, but are not limited to capping layers, elastomeric layers, release layers, barrier layers, and combinations thereof.
A flexographic printing element is produced by imaging the flexographic printing element to produce a relief image on a surface thereof, which is generally accomplished by selectively exposing the layer or layers of photopolymer to actinic radiation. The flexographic printing element is selectively exposed to actinic radiation in various ways to selectively cure portions of the layer or layers of photopolymer.
Following overall exposure to actinic radiation, the flexographic printing element is “developed” to remove unpolymerized areas of the layer of photopolymer and thereby form a relief image. The developing step removes portions of the at least one layer of photopolymer in the areas which were not exposed to actinic radiation, i.e., the unexposed areas or uncured areas of the layer of photopolymer. The development step may be accomplished, for example, by using a solvent to wash away uncured photopolymer (i.e., “solvent development”) or by softening the uncured photopolymer using heat to so that it may be removed (i.e., “thermal development”).
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images, in order to go from plate to press more quickly. Processes have been developed whereby flexographic relief image printing elements are prepared from flexographic printing elements using heat. The basic parameters of this process are known, as described in U.S. Pat. Nos. 7,241,124, 7,122,295, 6,773,859, 5,279,697, 5,175,072 and 3,264,103, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of the process allow for use of the process in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
Thermal development of the imaged and exposed flexographic printing element involves heating the flexographic printing element having at least one layer of photopolymer to a temperature sufficient to cause the uncured portions of the layer of photopolymer to liquefy, i.e., soften or melt or flow, and removing the uncured portions. The at least one layer of photopolymer is capable of partially liquefying upon thermal development. That is, during thermal development the uncured layer of photopolymer must soften or melt at a reasonable processing or developing temperature. The polymerized areas (cured portions) of the at least one layer of photopolymer have a higher melting temperature than the polymerized areas (uncured portions) and therefore do not melt, soften, or flow at the thermal development temperatures. The uncured portions can then be removed from the cured portions of the layer of photopolymer by contacting with an absorbent blotting material.
The composition of the photopolymer is such that there exists a substantial difference in the melt temperature between the cured and uncured polymer. It is precisely this difference that allows the creation of an image in the photopolymer when heated. The uncured photopolymer (i.e., the portions of the photopolymer not contacted with actinic radiation) melts or substantially softens while the cured photopolymer remains solid and intact at the temperature chosen. Thus the difference in melt temperature allows the uncured photopolymer to be selectively removed thereby creating a relief image.
During the thermal developing step, the at least one layer of photopolymer is heated by conduction, convection, radiation, or other heating methods to a temperature sufficient to effect melting of the uncured portions but not so high as to effect distortion of the cured portions of the layer. Typically, the printing element is heated to a surface temperature above about 40° C., typically from about 40° C. to about 230° C. in order to effect melting or flowing of the uncured portions of the layer of photopolymer. By maintaining more or less intimate contact of the blotting material with the layer of photopolymer that is molten in the uncured regions, a transfer of the uncured photopolymer from the layer of photopolymer to the blotting material takes place. While still in the heated condition, the blotting material is separated from the cured layer of photopolymer in contact with the support layer to reveal the relief structure. A cycle of the steps of heating the at least one layer of photopolymer and contacting the molten (portions) layer with the blotting material can be repeated as many times as necessary to adequately remove the uncured material and create sufficient relief depth, typically about 5 to 15 cycles.
One problem with current blotting techniques is that thermally developed printing plates may be vulnerable to high surface roughness (SR) due to the blotting materials used to remove uncured photopolymer. In addition to removing uncured photopolymer, blotting materials may embed patterns of the blotting material in the cured photopolymer relief. In other words, if the surface roughness of the blotting material is excessive, it may print blotter patterns, especially on the solid areas, leading to inconsistent ink coverage and low solid ink density (SID).
It is also a known problem in flexographic printing, that solid areas (i.e., areas in the image where there are no half tone dots), appear to print with less saturation and somewhat less uniformity than halftone areas representing dark image areas. Thus an area with a dot coverage of 95% to 98% may appear darker than a solid area (100%). A problem in printing solid areas in flexography is uneven ink transfer over the full solid image area, lack of density and a halo effect (i.e., a darker border) along the edges of the solid image area.
The level of color saturation achievable during flexographic printing is dependent upon many factors, prominent among which is the amount and uniformity of ink which can be applied to the print substrate, particularly in solid areas. This is commonly referred to as “Solid Ink Density” (SID). SID is sometimes higher at tone levels less than 100%, e.g., the optical print density achieved at the 97% tone level is slightly higher than that achieved at a 100% (solid) tone.
Therefore, it is desirable to provide improved methods of controlling and/or minimizing the surface roughness of thermally developed relief image printing elements as well as increasing the achievable solid ink density on solid areas of relief image printing elements.